Aerosol-generating device and aerosol-generating system with inductive heating system with efficient power control

ABSTRACT

An aerosol-generating device is provided, including one or more DC power sources; a load network including an inductor and a capacitor connected in series; first drive circuitry connected to the DC power source(s) and across the network and being configured to provide a first voltage drop across the network; second drive circuitry connected to the DC power source(s) and across the network and being configured to provide a second voltage drop across the network, the second voltage drop being in an opposite direction to the first voltage drop; and a controller connected to the first and the second drive circuitry and being configured to control the first and the second drive circuitry so that both the first and the second voltage drops are provided across the network periodically and so that the second voltage drop is not provided across the network simultaneously with the first voltage drop.

The disclosure relates to aerosol-generating systems that operate byheating an aerosol-forming substrate. In particular the inventionrelates to aerosol-generating systems that use inductive heating.

One type of aerosol-generating system is a system that heats, but doesnot combust, tobacco or another nicotine containing aerosol-formingsubstrate, to generate an aerosol for inhalation. Typically in heatedtobacco systems the tobacco or other aerosol-forming substrate is heatedby one or more electrically resistive heating elements that areconnected to a power supply. These systems need to be small enough to beeasily held during use and easily carried by a user between uses. Theyalso need to have their own internal power supply, which is typically asmall rechargeable battery.

More recently, there has been interest in using inductive heating toheat tobacco or nicotine containing aerosol-forming substrates inhandheld aerosol-generating systems. Inductive heating has a number ofpotential benefits. In particular, inductive heating allows theelectronic components to be separated from the aerosol-generatingsubstrate and the generated aerosol. This allows the system to be moreeasily cleaned and maintained and has potential benefits in terms of therobustness of the system.

Inductively heated systems operate by providing an inductor with a timevarying electrical voltage. This produces a time varying magnetic field,which in turn generates eddy currents and hysteresis losses in asusceptor material that is placed close to or in contact with theaerosol-forming substrate. Joule heating of the susceptor as a result ofthe induced currents heats the aerosol-forming substrate to produce anaerosol.

One problem with an inductively heated system that is powered by a smallbattery is ensuring that sufficient power is delivered to the inductorto generate the required heat in the susceptor. It would be desirable totransfer power to the inductor as efficiently as possible and toincrease the power transferrable to the inductor.

In one aspect of the invention, there is provided an aerosol-generatingdevice comprising:

one or more direct current (DC) power sources;

a load network comprising an inductor and a capacitor connected inseries;

first drive circuitry connected to the one or more DC power sources andconnected across the load network and configured to provide a firstvoltage drop across the load network;

second drive circuitry connected to the one or more DC power sources andconnected across the load network and configured to provide a secondvoltage drop across the load network, the second voltage drop being inan opposite direction to the first voltage drop; and

a controller connected to the first and second drive circuitry andconfigured to control the first and second drive circuitry so that boththe first and second voltage drops are provided across the load networkperiodically and so that the second voltage drop is not provided acrossthe load circuit simultaneously with the first voltage drop. -o Thefirst driving circuity may have a high voltage side and a low voltageside. The high voltage side may be connected to a first side of the loadnetwork. The low voltage may be connected to a second side of the loadnetwork. The first driving circuitry is configured to provide a firstvoltage drop across the load network. The second driving circuitry mayalso have a high voltage side and a low voltage side. The low voltageside of the second drive circuitry may be connected to first side of theload network. The high voltage side may be connected to the second sideof the load network. The second driving circuitry is configured toprovide a second voltage drop. The second voltage drop is in an oppositedirection to the first voltage drop.

The load network may have a first terminal on a first side and a secondterminal on a second side. The first drive circuitry connected to theone or more DC power sources may be connected to the first and secondterminals of the load network such that a positive DC voltage from theone or more DC power sources is applied to the first terminal of theload network. This results in a first voltage drop across the loadnetwork. The second drive circuitry connected to the one or more DCpower sources may be connected to the first and second terminals of theload network such that a positive DC voltage from the one or more DCpower sources is applied to the second terminal of the load network.This results in a second voltage drop across the load network. Thesecond voltage drop across the load network is in an opposite directionto the first voltage drop.

For inductive heating it is necessary to provide a time varying voltageacross the inductor. The arrangement of first and second drive circuitryalternately supplying voltage drops in different directions across theload network provides a time varying voltage and allows for efficientuse of power supplied by the power source or sources.

Advantageously, the controller is configured so that the first voltagedrop is provided periodically with a first frequency and so that thesecond voltage drop is provided periodically with substantially the samefrequency. “Substantially the same frequency” in this context meanswithin a few percent of the first frequency, and advantageously within 2percent of the first frequency. The first and second voltages can thenbe simply supplied with no overlap between them. The controller may beconfigured to provide the second voltage directly out of phase with thefirst voltage.

The first frequency may be a high frequency. In this context, “highfrequency” is to be understood to mean a frequency in the range fromabout 100 Kilohertz (khz) to about 30 Megahertz (MHz). The firstfrequency may be greater than 1 Mega Hertz. The first frequency may beless than 10 Megahertz. Preferably, the first frequency is in the rangebetween 5 Megahertz and 7 Megahertz.

The first drive circuitry and second drive circuitry may be comprised ofright and left side driving means. The circuit components connected toone end of the load network may form the right side driving means andthe circuit components connected to the other end of the load networkmay form the left side driving means. The first drive circuitry maycomprise circuit components from both the right and left side drivingmeans. The second drive circuitry may comprise circuit components fromboth the right and left side driving means. The right and left sidedriving means may each comprise a switching circuit, which may be aresonant switching circuit. The right side driving means together withthe load network may form a first power amplifier. The left side drivingmeans together with the load network may comprise a second poweramplifier. The first power amplifier may be a D-class amplifier. Thesecond power amplifier may be a D-class amplifier. The first poweramplifier may be an E-class amplifier. The second power amplifier may bean E-class amplifier.

The controller may be configured to provide the first voltage drop as asquare waveform voltage. The controller may be configured to provide thesecond voltage drop as a square waveform voltage. The first voltage dropmay be provided with the same or a different duty cycle to the secondvoltage drop. Advantageously the controller is configured to provide adead time period of a least a few nanoseconds between the end of onevoltage drop and the start of the next voltage drop in the oppositedirection, in order to avoid burn out of associated switches in thedrive circuitry.

The one or more DC power sources may comprise a single battery connectedto both the first and the second drive circuitry. The battery may be arechargeable battery. The battery may be a lithium ion battery, forexample a Lithium-Cobalt, a Lithium-Iron-Phosphate, Lithium Titanate ora Lithium-Polymer battery. Alternatively, the battery may another formof rechargeable battery, such as a Nickel-metal hydride battery or aNickel cadmium battery.

Alternatively, the one or more DC power sources may comprise twobatteries, with one battery connected to the first drive circuitry andanother battery connected to the second drive circuitry. The one or moreDC power sources may comprise two batteries connected in series withelectrical ground being defined between the two batteries so that onebattery provides a positive voltage and the other provides a negativevoltage.

The controller may comprise a microcontroller. The microcontroller maybe any suitable microcontroller but is preferably programmable.

The device may comprise a housing containing the one or more DC powersources, the load network, the first and second drive circuitry and thecontroller, the housing defining a cavity for receiving anaerosol-forming substrate. The device may be configured to inductivelyheat the aerosol-forming substrate.

The inductor may be a coil positioned adjacent to or surrounding thecavity. In one embodiment, the inductor is a helical coil that surroundsat least a portion of the cavity. Alternatively, the inductor may beflat spiral inductor coil positioned adjacent to the side or the base,or both the side and the base, of the cavity. The inductor should bepositioned to provide a time varying magnetic field in a susceptormaterial configured to heat an aerosol-forming substrate in use.

The device may comprise a plurality of inductors configured to beactivated at different times during operation of the device. Theplurality of inductors may be positioned to provide spatially separate(or spatially partially overlapping) time varying magnetic fields sothat different portions of an aerosol-forming substrate can be heated atdifferent times during operation. If the device comprises a plurality ofinductors, then the device may comprise a plurality of first and seconddrive circuits.

As used herein, an ‘aerosol-generating device’ relates to a device thatinteracts with an aerosol-forming substrate to generate an aerosol. Theaerosol-forming substrate may be part of an aerosol-generating article.An aerosol-generating device may be a device that interacts with anaerosol-forming substrate of an aerosol-generating article to generatean aerosol that is directly inhalable into a user's lungs thorough theuser's mouth. The aerosol-forming substrate may be fully or partiallycontained within the device.

In another aspect of the invention, there is provided anaerosol-generating system comprising an aerosol-generating device inaccordance with any one of the preceding claims and anaerosol-generating article comprising an aerosol-forming substrate,wherein the aerosol-generating article is configured to be received atleast partially within the aerosol-generating device.

The aerosol-forming substrate may be a solid aerosol-forming substrate.Alternatively, the aerosol-forming substrate may be a liquid or maycomprise both solid and liquid components or may comprise a gel. Theaerosol-forming substrate may comprise a tobacco-containing materialcontaining volatile tobacco flavour compounds which are released fromthe substrate upon heating. Alternatively, the aerosol-forming substratemay comprise a non-tobacco material. The aerosol-forming substrate mayfurther comprise an aerosol former. Examples of suitable aerosol formersare glycerine and propylene glycol.

If the aerosol-forming substrate is a solid aerosol-forming substrate,the solid aerosol-forming substrate may comprise, for example, one ormore of: powder, granules, pellets, shreds, strips or sheets containingone or more of: herb leaf, tobacco leaf, fragments of tobacco ribs,reconstituted tobacco, homogenised tobacco, extruded tobacco, cast leaftobacco and expanded tobacco. The solid aerosol-forming substrate may bein loose form, or may be provided in a suitable container or cartridge.Optionally, the solid aerosol-forming substrate may contain additionaltobacco or non-tobacco volatile flavour compounds, to be released uponheating of the substrate. The solid aerosol-forming substrate may alsocontain capsules that, for example, include the additional tobacco ornon-tobacco volatile flavour compounds and such capsules may melt duringheating of the solid aerosol-forming substrate.

Optionally, the solid aerosol-forming substrate may be provided on orembedded in a thermally stable carrier. The carrier may take the form ofpowder, granules, pellets, shreds, spaghettis, strips or sheets.Alternatively, the carrier may be a tubular carrier having a thin layerof the solid substrate deposited on its inner surface, or on its outersurface, or on both its inner and outer surfaces. Such a tubular carriermay be formed of, for example, a paper, or paper like material, anon-woven carbon fibre mat, a low mass open mesh metallic screen, or aperforated metallic foil or any other thermally stable polymer matrix.

The solid aerosol-forming substrate may be deposited on the surface ofthe carrier in the form of, for example, a sheet, foam, gel or slurry.The solid aerosol-forming substrate may be deposited on the entiresurface of the carrier, or alternatively, may be deposited in a patternin order to provide a non-uniform flavour delivery during use.

Although reference is made to solid aerosol-forming substrates above, itwill be clear to one of ordinary skill in the art that other forms ofaerosol-forming substrate may be used with other embodiments. Forexample, the aerosol-forming substrate may be a liquid aerosol-formingsubstrate. If a liquid aerosol-forming substrate is provided, theaerosol-generating device preferably comprises means for retaining theliquid. For example, the liquid aerosol-forming substrate may beretained in a container. Alternatively or in addition, the liquidaerosol-forming substrate may be absorbed into a porous carriermaterial. The porous carrier material may be made from any suitableabsorbent plug or body, for example, a foamed metal or plasticsmaterial, polypropylene, terylene, nylon fibres or ceramic. The liquidaerosol-forming substrate may be retained in the porous carrier materialprior to use of the aerosol-generating device or alternatively, theliquid aerosol-forming substrate material may be released into theporous carrier material during, or immediately prior to use. Forexample, the liquid aerosol-forming substrate may be provided in acapsule. The shell of the capsule preferably melts upon heating andreleases the liquid aerosol-forming substrate into the porous carriermaterial. The capsule may optionally contain a solid in combination withthe liquid. Alternatively, the carrier may be a non-woven fabric orfibre bundle into which tobacco components have been incorporated. Thenon-woven fabric or fibre bundle may comprise, for example, carbonfibres, natural cellulose fibres, or cellulose derivative fibres.

During operation, the aerosol-forming substrate may be completelycontained within the aerosol-generating device. In that case, a user maypuff on a mouthpiece of the aerosol-generating device. Alternatively,during operation an aerosol-forming article containing theaerosol-forming substrate may be partially contained within theaerosol-generating device. In that case, the user may puff directly onthe aerosol-forming article.

The aerosol-forming article may be substantially cylindrical in shape.The aerosol-forming article may be substantially elongate. Theaerosol-forming article may have a length and a circumferencesubstantially perpendicular to the length. The aerosol-forming substratemay be substantially cylindrical in shape. The aerosol-forming substratemay be substantially elongate. The aerosol-forming substrate may alsohave a length and a circumference substantially perpendicular to thelength.

The aerosol-forming article may have a total length betweenapproximately 30 mm and approximately 100 mm. The aerosol-formingarticle may have an external diameter between approximately 5 mm andapproximately 12 mm. The aerosol-forming article may comprise a filterplug. The filter plug may be located at the downstream end of theaerosol-forming article. The filter plug may be a cellulose acetatefilter plug. The filter plug is approximately 7 mm in length in oneembodiment, but may have a length of between approximately 5 mm toapproximately 10 mm.

In one embodiment, the aerosol-forming article has a total length ofapproximately 45 mm. The aerosol-forming article may have an externaldiameter of approximately 7.2 mm. Further, the aerosol-forming substratemay have a length of approximately 10 mm. Alternatively, theaerosol-forming substrate may have a length of approximately 12 mm.Further, the diameter of the aerosol-forming substrate may be betweenapproximately 5 mm and approximately 12 mm. The aerosol-forming articlemay comprise an outer paper wrapper. Further, the aerosol-formingarticle may comprise a separation between the aerosol-forming substrateand the filter plug. The separation may be approximately 18 mm, but maybe in the range of approximately 5 mm to approximately 25 mm.

The device is preferably a portable or handheld device that iscomfortable to hold between the fingers of a single hand. The device maybe substantially cylindrical in shape and has a length of between 70 and200 mm. The maximum diameter of the device is preferably between 10 and30 mm. In one embodiment the device has a polygonal cross section andhas a protruding button formed on one face. In this embodiment, thediameter of the device is between 12.7 and 13.65 mm taken from a flatface to an opposing flat face; between 13.4 and 14.2 taken from an edgeto an opposing edge (i.e., from the intersection of two faces on oneside of the device to a corresponding intersection on the other side),and between 14.2 and 15 mm taken from a top of the button to an opposingbottom flat face.

The aerosol-generating article may comprise a susceptor element orelements. Alternatively, or in addition, the aerosol generating devicemay comprise a susceptor element or elements. As used herein, a“susceptor element” means a conductive element that heats up whensubjected to a changing magnetic field. This may be the result of eddycurrents induced in the susceptor element and/or hysteresis losses.Advantageously the susceptor element comprises a ferromagnetic material.

The susceptor element is advantageously in thermal proximity to theaerosol-forming substrate in use, so that heat generated in thesusceptor can be transferred by conduction or convection to theaerosol-forming substrate in order to generate an aerosol.

The material and the geometry for the susceptor element can be chosen toprovide a desired electrical resistance and heat generation.Advantageously, the susceptor element has a relative permeabilitybetween 1 and 40000. When a reliance on eddy currents for a majority ofthe heating is desirable, a lower permeability material may be used, andwhen hysteresis effects are desired then a higher permeability materialmay be used. Preferably, the material has a relative permeabilitybetween 500 and 40000. This provides for efficient heating.

The material of the susceptor element may be chosen because of its Curietemperature. Above its Curie temperature a material is no longerferromagnetic and so heating due to hysteresis losses no longer occurs.In the case the susceptor element is made from one single material, theCurie temperature may correspond to a maximum temperature the susceptorelement should have (that is to say the Curie temperature is identicalwith the maximum temperature to which the susceptor element should beheated or deviates from this maximum temperature by about 1-3%). Thisreduces the possibility of rapid overheating.

If the susceptor element is made from more than one material, thematerials of the susceptor element can be optimized with respect tofurther aspects. For example, the materials can be selected such that afirst material of the susceptor element may have a Curie temperaturewhich is above the maximum temperature to which the susceptor elementshould be heated. This first material of the susceptor element may thenbe optimized, for example, with respect to maximum heat generation andtransfer to the aerosol-forming substrate to provide for an efficientheating of the susceptor on one hand. However, the susceptor element maythen additionally comprise a second material having a Curie temperaturewhich corresponds to the maximum temperature to which the susceptorshould be heated, and once the susceptor element reaches this Curietemperature the magnetic properties of the susceptor element as a wholechange. This change can be detected and communicated to amicrocontroller which then interrupts the operation of the drivecircuitry until the temperature has cooled down below the Curietemperature again, whereupon operation of the drive circuitry can beresumed.

The susceptor element may be in the form of a mesh. If theaerosol-forming substrate is a liquid, the mesh may be configured toallow the liquid to form a meniscus in the interstices of the meshsusceptor element. This provides for efficient heating of theaerosol-forming substrate. As used herein the term “mesh” encompassesgrids and arrays of filaments having spaces therebetween, and mayinclude woven and non-woven fabrics. The mesh may comprise a pluralityof ferrite filaments. The filaments may define interstices between thefilaments and the interstices may have a width of between 10 μm and 100μm. Preferably the filaments give rise to capillary action in theinterstices, so that in use, liquid to be vapourised is drawn into theinterstices, increasing the contact area between the susceptor elementand the liquid.

Embodiments of the invention will now be described in detail, withreference to the accompanying drawings, in which:

FIG. 1 shows an embodiment of an aerosol-generating system comprising anaerosol-generating device and an aerosol-generating article, inaccordance with an embodiment of the invention;

FIG. 2 is a schematic illustration of the components of the electricalcomponents of the system shown in FIG. 1;

FIG. 3 is a schematic illustration of the power supply electronics inaccordance with the invention;

FIG. 4 illustrates the voltages applied to opposite sides of the loadcircuit by the drive circuitry;

FIG. 5a illustrates an arrangement of the power supply electronics inaccordance with an embodiment of the invention;

FIG. 5b illustrates the components of the power supply circuit of FIG.5a through which current passes during a first time period;

FIG. 5c illustrates the components of the power supply circuit of FIG.5a through which current passes during a second time period; and

FIG. 6 illustrates an alternative arrangement of the power supplyelectronics.

FIG. 1 shows an embodiment of an aerosol-generating system comprising aninductive heating device 1 according to the invention. The inductiveheating device 1 comprises a device housing 10, which can be made ofplastic, and a DC power source comprising a rechargeable battery 110.Inductive heating device 1 further comprises a docking port 12comprising a pin 120 for docking the inductive heating device to acharging station or charging device for recharging the rechargeablebattery 110. Still further, inductive heating device 1 comprises a powersupply electronics 13 which is configured to operate at a desiredfrequency.

Power supply electronics 13 is electrically connected to therechargeable battery 110 through a suitable electrical connection 130.And while the power supply electronics 13 comprises additionalcomponents which cannot be seen in FIG. 1, it comprises in particular anLC load network which in turn comprises an inductor L, this beingindicated by the dashed lines in FIG. 1. Inductor L is embedded in thedevice housing 10 at the proximal end of device housing 10 to surround acavity 14 which is also arranged at the proximal end of the devicehousing 10.

Inductor L may comprise a helically wound cylindrical inductor coilhaving a cylindrical shape. The helically wound cylindrical inductorcoil L may have a diameter d in the range of about 5 mm to about 10 mm,and in particular the diameter d may be about 8 mm. The length 1 of thehelically wound cylindrical inductor coil may be in the range of about0.5 mm to about 18 mm. The inner volume accordingly, may be in the rangeof about 0.015 cm³ to about 1.3 cm³.

The aerosol-forming substrate 20 comprises a susceptor 21 and isaccommodated in the cavity 14 at the proximal end of the device housing10 such that during operation the inductor L (the helically woundcylindrical inductor coil) is inductively coupled to the susceptor 21 ofthe aerosol-forming substrate 20 of smoking article 2.

The susceptor 21 does not necessarily have to form part of theconsumable, but it could be part of the device itself. It is alsopossible to have susceptor elements in both the device and in theconsumable.

A filter portion 22 of the smoking article 2 may be arranged outside thecavity 14 of the inductive heating device 1 so that during operation theconsumer may draw the aerosol through the filter portion 22. Once thesmoking article is removed from the cavity 14, the cavity 14 can beeasily cleaned, since except for the open distal end through which theaerosol-forming substrate 20 of the smoking article 2 is to be inserted,the cavity is fully closed and surrounded by the inner walls of theplastic device housing 10 defining the cavity 14.

FIG. 2 shows a block diagram of an embodiment of the aerosol-deliverysystem comprising the inductive heating device 1 according to theinvention, with some optional aspects or components as will be discussedbelow. Inductive heating device 1 together with the aerosol-formingsubstrate 20 comprising the susceptor 21 forms an embodiment of theaerosol-delivery system according to the invention. The block diagramshown in FIG. 2 is an illustration taking the manner of operation intoaccount. As can be seen, the inductive heating device 1 comprises a DCpower source 11 (in FIG. 1 comprising the rechargeable battery 110),control electronics (microprocessor control unit) 131, a DC/AC converter132 (embodied as a DC/AC inverter), a matching network 133 foradaptation to the load, and the inductor L. Control electronics 131,DC/AC converter 132 and matching network 133 as well as inductor L areall part of the power supply electronics 13 (see FIG. 1).

The DC supply voltage (VDC) and the DC current (IDC) drawn from the DCpower source 11 are provided by feed-back channels to the microprocessorcontrol unit 131, preferably by measurement of both the DC supplyvoltage (VDC) and the DC current (IDC) drawn from the DC power source11, to control the further supply of AC power to the LC load network.The matching network 133 may be provide for optimal adaption to theload, but it is not essential and is not included in the description ofthe following detailed examples.

FIG. 3 shows some essential components of the power supply electronics13 and in particular the DC/AC inverter 132. The power electronics 13comprises a load branch 1320 which in turn comprises a LC load network1323 configured to operate at a low load R 1324. The resistance R 1234shown in FIG. 3 is not a real component; it is an equivalent seriesresistance, of the susceptor in the coil. The LC load network comprisesa capacitor C and an inductor L (having an ohmic resistance Rcoil)connected in series. The LC load network 1323 is inductively coupled tothe susceptor during operation.

In this embodiment, the DC/AC inverter comprises a left driving means Diand a right driving means D₂ connected to opposite ends of the loadnetwork 1320. Each of the left driving means and right driving means isconnected to the DC power source and to the load network 1320 which hasa first terminal on a first side and a second terminal on a second side.In FIG. 3 two separate DC power sources are depicted, but typically theleft and right driving means are both connected to the same powersource.

The left driving means D₁ is configured to provide a first periodicwaveform voltage V_(R) to the load branch 1320, with a selectedfrequency F, and having an amplitude ranging from a first value to asecond value lower than the first value. In a similar manner, rightdriving means D₂ is configured to provide a second waveform voltageV_(L) to the load branch 1320, having substantially the same frequency Fas the first waveform voltage, and similarly having an amplitude rangingfrom a first respective value to a second respective value lower thanthe first value.

An example of the first and second periodic waveforms is illustratedschematically in

FIG. 4. It can be seen that the two waveforms are square waves that aredirectly out of phase (or in phase opposition) with one another. Becausethe square waves are applied from opposite ends of the load network,they provide voltage drops across the load network in oppositedirections. The voltage drops are of opposite polarity to one another,where opposite polarity in this context refers to the relative positionof the high and low voltage sides, rather than requiring a positivevoltage and a negative voltage. By applying voltage pulses alternatelyfrom either side of the load network in this way, an AC voltage iseffectively supplied to the inductor and power can be efficientlydissipated in the load network, and in particular in the susceptorelement.

There are a number of ways in which the arrangement shown in FIG. 3 canbe implemented to provide a voltage profile as illustrated in FIG. 4.FIG. 5a illustrates a first embodiment, in which the right and leftdriving means together with the load network form Class-D amplifiers. Inparticular, each of the driving means comprise a pair of transistorswitches T₁, T₂, and T₃, T₄ connected to the DC power source in series.The load network 1323 is connected to the left driving means at aposition between the two transistor switches T₁ and T₂. The load network1323 is connected to the right driving means at a position between thetwo transistor switches T₃ and T₄. The load network is effectivelyshared between the two Class-D amplifiers.

The transistor switches are Field Effect Transistors (FETs) andcontrolled by the control electronics to supply waveform as illustratedin FIG. 4. The control electronics supplies a high frequency alternatingswitching voltage 1321, 1322, 1325, 1326 to the gate of each of thetransistors so that during one half period the transistors T₁ and T₃ areconducting and transistors T₂ and T₄ are off, and during the other halfperiod transistors T₂ and T₄ are conducting and transistors T₁ and T₃are off. FIG. 5b illustrates the connection of the inductor L to thepower supply during the first half period, with transistors T₁ and T₃conducting. The arrangement shown in FIG. 5b can be considered tocomprise first drive circuitry that operates to supply the load networkwith a first periodic voltage drop. FIG. 5c illustrates the connectionof the inductor L to the power supply during the second half period,with transistors T₂ and T₄ conducting. The arrangement shown in FIG. 5ccan be considered to comprise second drive circuitry that operates tosupply the load network with a second periodic voltage drop, at the samefrequency as the first periodic voltage drop but of opposite polarityand directly out of phase with the first periodic voltage.

It is of course possible to provide periodic voltage drops that aredifferent to those shown in FIG. 4. In particular, the waveforms mayhave a duty cycle of less than 50%. It will be appreciated that voltagepulses V_(R) and V_(L) preferably do not overlap each other in time, inorder to avoid high and potentially damaging current going throughtransistors T₁ and T₂ or T₃ and T₄.

FIG. 6 illustrates an alternative arrangement for implementing thetopology shown in FIG. 3, using an Class-E amplifier topology in placeof a Class-D amplifier topology. In the arrangement of FIG. 6, the leftside driving means together with the load network forms a first Class-Eamplifier and the right side driving means together with the loadnetwork forms a second Class-E amplifier. Each Class-E amplifiercomprises a single FET switch. The switch T₅ in the left side drivingmeans is controlled by a high frequency switching voltage 1327 and theswitch T₆ in the right side driving means is controlled by a highfrequency switching voltage 1328. The switching voltages 1327 and 1328are out of phase with one another to provide the two, directly out ofphase periodic voltage waveforms V_(L) and V_(R), as exemplified in FIG.4.

It should be clear that other forms of drive circuitry are possible. Forexample, it is possible to have an arrangement using the right sidedriving means shown in FIG. 5a and the left side driving means shown inFIG. 6, or using the left side driving means shown in FIG. 5a and theright side driving means shown in FIG. 6. Other forms of resonantswitching circuitry may be employed as the right and left side drivingmeans too.

1.-15. (canceled)
 16. An aerosol-generating device, comprising: one ormore DC power sources; a load network comprising an inductor and acapacitor connected in series; first drive circuitry connected to theone or more DC power sources and connected across the load network andbeing configured to provide a first voltage drop across the loadnetwork; second drive circuitry connected to the one or more DC powersources and connected across the load network and being configured toprovide a second voltage drop across the load network, the secondvoltage drop being in an opposite direction to the first voltage drop;and a controller connected to the first and the second drive circuitryand being configured to control the first and the second drive circuitryso that both the first and the second voltage drops are provided acrossthe load network periodically and so that the second voltage drop is notprovided across the load network simultaneously with the first voltagedrop.
 17. The aerosol-generating device according to claim 16, whereinthe controller is further configured so that the first voltage isprovided periodically with a first frequency and so that the secondvoltage is provided periodically at substantially a same frequency asthe first frequency.
 18. The aerosol-generating device according toclaim 16, wherein the first drive circuitry and the second drivecircuitry are composed of right and left side driving means, whereincircuit components connected to one end of the load network form theright side driving means and circuit components connected to the otherend of the load network form the left side driving means, and whereineach of the right and the left side driving means comprise a switchingcircuit.
 19. The aerosol-generating device according to claim 18,wherein the right side driving means together with the load networkforms a first power amplifier, and wherein the left side driving meanstogether with the load network forms a second power amplifier.
 20. Theaerosol-generating device according to claim 19, wherein the first poweramplifier or the second power amplifier, or both the first poweramplifier and the second power amplifier, is a D-class amplifier. 21.The aerosol-generating device according to claim 19, wherein the firstpower amplifier or the second power amplifier, or both the first poweramplifier and the second power amplifier, is an E-class amplifier. 22.The aerosol-generating device according to claim 16, wherein thecontroller is further configured to provide the second voltage directlyout of phase with the first voltage.
 23. The aerosol-generating deviceaccording to claim 16, wherein the one or more DC power sourcescomprises a battery connected to both the first and the second drivecircuitry.
 24. The aerosol-generating device according to claim 23,wherein the battery is a rechargeable battery.
 25. Theaerosol-generating device according to claim 16, further comprising ahousing containing the one or more DC power sources, the load network,the first and the second drive circuitry, and the controller, whereinthe housing defines a cavity configured to receive an aerosol-formingsubstrate, and wherein the aerosol-generating device is configured toinductively heat the aerosol-forming substrate.
 26. Theaerosol-generating device according to claim 25, wherein the inductor isa coil positioned adjacent to or surrounding the cavity.
 27. Theaerosol-generating device according to claim 16, wherein theaerosol-generating device is a handheld device.
 28. Anaerosol-generating system comprising an aerosol-generating deviceaccording to claim 16 and an aerosol-generating article comprising anaerosol-forming substrate, wherein the aerosol-generating article isconfigured to be received at least partially within theaerosol-generating device.
 29. The aerosol-generating system accordingto claim 28, wherein the aerosol-generating article comprises asusceptor material.
 30. The aerosol-generating system according to claim28, wherein the aerosol-forming substrate comprises a tobacco-containingmaterial containing volatile tobacco flavour compounds, which arereleased from the substrate upon heating.